Recent studies in our laboratory have shown that in vitro maintenance of isolated rat islets at 24 degrees C for seven days in conjunction with a single injection of antilymphocyte serum (ALS) into diabetic recipients will produce marked prolongation (greater than 100 days) of survival of islet allografts across a major histocompatibility barrier. Rejection of the allografts is initiated by the intravenous injection of peritoneal exudate cells. Studies are in progress to determine the minimum number of peritoneal exudate cells required to induce rejection and to identify the type or types of cells required to initiate immune rejection. Studies are in progress to attempt to remove passenger leucocytes in vitro by the culture of isolated islets in the presence of ALS, carageenan and hyperbaric oxygen and to determine the effect of these in vitro procedures on prolonging islet allograft survival. The effect of using dispersed cell cultures of isolated islets for removal of passenger leucocytes will also be determined. The present pretreatment regimen of in vitro culture at 24 degrees C and a single injection of ALS into the recipient will be used for studies on attempting to prolong survival of xenografts of mouse or hamster islets transplanted into diabetic rats.